Vortex premixer for combustion apparatus

ABSTRACT

In one embodiment, a premixer is provided for use, e.g., with trapped vortex (TVC) combustors, that injects premixed fuel and air directly into the recirculating vortex, in a manner compatible with natural vortex flow in the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit pursuant to 35 U.S.C. §119(e) ofU.S. provisional patent application 61/241,940, filed Sep. 13, 2009, theentire disclosure of which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the fields of combustion and gas turbines, andespecially gas turbine designs for dry low emissions, and moreparticularly concerns apparatus and methods for premixing fuel and airto achieve ultra low combustion emissions.

2. Description of the Related Art

A premixer can be useful in enhancing flame stability in an energyrelease/conversion system, such as a combustor for powering a gasturbine engine or generator. For purposes of brevity, this disclosureuses the term “combustor” throughout, but it should be understood thatthe disclosure relates more generally to energy release/-conversionsystems that can be operated as either a combustor or a fuel reformer,depending on operating conditions, as well as dedicated combustor andreformer apparatus. Unless the context clearly requires otherwise, theterms “combustor”, “reformer” and “energy release/conversion system”should be regarded as completely interchangeable.

Premixers currently employed in the art include “hub and spoke”configured units, such as those employed by General Electric, Pratt &Whitney, Siemens, etc., placed at the inlet of a combustor, with fuelsupplied through the hub and injected out of radial spokes and/orintegrated into swirler vanes. The spokes of these premixers have aplurality of uniform sized axial holes transverse to the combustorinlet. The design of these premixers tends to optimize them for aparticular fuel-air momentum flux ratio. Accordingly, these premixerswork best in a narrow power band, and do not provide the most uniformfuel-air mixture over the entire engine operating envelope. The systemwould generate either too lean or too rich fuel-air zones, adverselyimpacting emissions. It appears, therefore, that further improvement inthe operation of energy release/conversion systems might be possible byredesigning the premixing apparatus used in conjunction with thesesystems to perform better over a broader range of operating conditions.

In one class of combustion apparatus, known as “trapped vortex”combustors (TVCs, as addressed at further length later in thisdisclosure), a cavity is provided in the combustion area, for example,between bluff bodies or in the wall of the combustor, in which vorticesand/or other turbulence will form, in order to stabilize combustion withlean mixtures. See for example U.S. Pat. No. 5,857,339 to Roquemore etal. Fuel and/or air may be injected into a trapped vortex cavity throughdiscrete injectors, to induce greater mixing in this area and furtherpromote flame stability. The discrete fuel and air injectors may besituated, for example, on the forward and aft walls of a trapped vortexarea defined by the walls of the combustion cavity. See, e.g., Burrus,U.S. Pat. No. 5,791,148.

Haynes, et al., GE Global Research, “Advanced Combustion Systems forNext Generation Gas Turbines, Final Report”, January 2006(DE-FC26-01NT41020), describes a combustor similar in layout to thecombustor described by Burrus. In certain embodiments disclosed byHaynes et al., as an alternative to discrete fuel and air inlets, aspreviously practiced, fuel and air may be premixed and introducedthrough the inlet cone and/or through the forward or aft walls of thecombustion cavity. Embodiments in which premixture was introduced bothinto the inlet cone and the combustion cavity resulted in the creationof a stacked double vortex, with highly turbulent mixing.

Steele, et al., in U.S. Pat. No. 7,603,841, describing another TVCembodiment, discloses a combustor having inlet premixing as well as aftinjectors into a combustion cavity defined in part by a bluff body. Inthis embodiment, the aft injectors are pointed in a direction oppositeto incoming premixture flow to induce turbulent vortex mixing.

Heretofore, all TVC designs that have injected fuel, air and/or premixedfuel and air into the combustion cavity have been designed to induceturbulence, to cause the formation of additional vortices, or otherwiseto increase turbulent mixing in the vortex cavity. For example, FIG. 3-7of Haynes et al. shows a dual trapped vortex in each TVC cavity. The“natural” flow that would occur in these cavities, meaning the fluidflow that would naturally occur in the cavities in the absence of thepremixture injection, given the flow otherwise taking place through themain flow path of the combustor, would be a single vortex. In the caseshown in FIG. 3-7 of Haynes et al., the second vortex in the “dualvortex” show is created by the injection of premixture into the TVCcavity, and would not otherwise exist. In other cases, for example,where only a single vortex is provided, the main vortex shown mightotherwise exist, but is substantially modified by the impact of theadded premixture, for example, by being moved translationally from itsnatural position in the cavity, made much more turbulent, or otherwisesubstantially distorted.

Commonly assigned U.S. Pat. Pub. 2008/0092544 A1, by Rakhmailov(Rakhmailov '544 publication), discloses a premixer deployed incombination with a combustor designed in accordance with the disclosureof commonly assigned U.S. Pat. No. 7,086,854 to Rakhmailov et al.(Rakhmailov '854). The premixer in the Rakhmailov '544 publication isdeployed only at the inlet of the combustor. The inlet of this combustorhas a high velocity of fluid flow, and thus the inlet premixing is donein a high-velocity environment.

While the design described in the Rakhmailov '544 publication adds aninlet premixer to the recirculating vortex combustor described inRakhmailov '854, neither disclosure contains any provision for injectionof fuel, air and/or premixed fuel and air directly into the vortexcavity. Indeed, Rakhmailov '854 expressly teaches away from admittingfuel into the hot recirculating gas in a recirculation vortex cavity,stating that turbulent mechanical mixing can reduce overallrecirculation velocity, result in nonuniform fuel distribution, andreduce temperatures where the recirculating flow joins the inlet flow,contrary to the design goals of Rakhmailov '854.

The entire respective disclosures of Roquemore et al., Burris, Haynes etal., Steele et al., Rakhmailov '854, and the Rakhmailov '544 publicationare each incorporated by reference herein in their entirety, for allpurposes.

It would be desirable to improve upon the prior art in a number ofrespects. First, it would be desirable to improve inlet premixers forany type of combustor by making the premixer more adaptable to a widerrange of operating conditions. Second, it would be desirable to providefor premixing in the vortex area of a combustor to enhance rather thandisrupt normal vortex flow. Third, it would be desirable to providemethods for using inlet and vortex premixers advantageously incombination with each other.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a superior fuel-airpre-mixing apparatus and method for use in combustors.

Desired attributes of such premixer apparatus include:

-   -   The premixer should provide uniform fuel distribution over the        cross sectional area of the combustor inlet.    -   The premixer should provide a uniform fuel-air mixture over a        broad range of engine operating conditions.    -   The premixer should provide a short premixing length.    -   The premixer should be compatible with a broad range of fuels,        including all gaseous and liquid fuels used in gas turbines.    -   The premixer should provide low emissions generation.    -   The premixer, or a separate premixer, should be adaptable to        assist in stabilizing combustion in the trapped vortex cavity of        a TVC, consistent with the flow patterns for which the TVC was        designed, including low-turbulence patterns, where those are        employed.    -   It should be possible to coordinate the operation of any        plurality of premixers employed in the design to provide the        best operation over the operating range of the system.    -   The premixer systems and methods provided should be applicable        to a broad range of applications.

In one embodiment, these objectives may be achieved by providing aninlet premixer assembly comprising a hub, a plurality of radial spokes,and a plurality of concentric, aerodynamic injector rings attached tothe spokes, with a plurality of radially-directed injection holes. Theinjection holes are provided with a plurality of different diameters,facilitating good mixing over a broad power range. Due to configurationand hole sizes, the assembly is gas and liquid compatible. The radial,concentric injection formation allows for a short injection path, due tothe greater number of fuel injection sites over the cross-sectionalarea.

In a second aspect, for use in conjunction with trapped-vortex reactordesigns, a further premixer embodiment may be provided that injectspremixed fuel and air directly into the trapped vortex cavity in amanner compatible with the flow patterns in the cavity for which thesystem was designed. When used in conjunction with a recirculatingvortex design, this premixer can be arranged so that the premixed fueland air joins the vortex flow tangentially in a smooth, continuousmanner and/or to reinforce combustion in the toroid. In other designs,premixture may be introduced at one or more TVC locations in one or moredirections consistent with local vortex flow in the area ofintroduction.

Methods may also provided for the two embodiments described above to beused in conjunction with each other, and their settings adjusted incoordination with each other, for fuel staging during operation.Extremely low emissions results have been achieved in this manner.

In other embodiments, alternative combustion cavity layouts are providedfor practicing fuel staging in a trapped vortex (TVC) combustionapparatus comprising an inlet premixer, for injecting fuel-air mixtureinto the inlet of the combustion apparatus and one or more vortexpremixers, for injecting fuel-air mixture into the recirculating vortexwithin each of one or more trapped vortex cavities. A plurality ofpremixer-fed TVC cavities, may, for example, be laid out axially,radially, peripherally, internally, or in combinations of sucharrangements. These layouts may be used in conjunction with a fuelstaging method whereby the relative proportion of mixture introducedthrough the inlet and the respective vortex premixers can be varied as afunction of operating conditions.

Combustion apparatus and methods in accordance with the invention may beused in all gas turbine applications, including without limitationland-based power generation, commercial jet aircraft engines, auxiliarypower units (APUs) for aircraft, in Integrated Gasification CombinedCycle (IGCC) plants, and in Combined Heat and Power (CHP) orcogeneration plants.

Other aspects and advantages of the invention will be apparent from theaccompanying drawings, and the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals represent like parts, in which:

FIG. 1 is a perspective cross-sectional view from the inlet side to theexit side of a can-annular combustor incorporating two differentpremixer devices in accordance with certain embodiments of theinvention.

FIG. 2A shows cross-sectional views of the inlet premixer shown in FIG.1, and FIG. 2B shows detailed cross-sectional views of spoke and ringelements comprising this premixer.

FIG. 3 are additional cross-sectional views of the inlet premixerillustrated in FIGS. 2A and 2B, also showing the placement of fuelinjection orifices in the injector rings.

FIG. 4 is a partial cutaway and perspective view from outside the toroidof the vortex premixer shown in FIG. 1.

FIG. 5 shows an alternate embodiment of a vortex premixer used incombination with a TVC having rectlinear walls in the trapped vortexcavity.

FIGS. 6A-6C show examples of fuel staging strategies in accordance withone embodiment of the invention, and FIGS. 6D-6E are correspondinggraphs of flame temperature vs. engine power.

FIGS. 7A-7D show a variety of combustor layouts incorporating aplurality of TVC cavities may be used in conjunction with individuallycontrollable inlet and vortex premixers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following is a detailed description of certain embodiments of theinvention chosen to provide illustrative examples of how it mayadvantageously be implemented. The scope of the invention is not limitedto the specific embodiments described, nor is it limited by any specificimplementation, composition, embodiment or characterization depicted inthe accompanying drawings or stated or described in the inventionsummary or the abstract. In addition, it should be noted that thisdisclosure describes a number of methods that each comprise a pluralityof steps. Nothing contained in this written description should beunderstood to imply any necessary order of steps in such methods, otherthan as specified by express claim language.

The present disclosure is applicable to any gas turbine combustor orreaction chamber. Certain aspects of this disclosure are relevant to anyenergy release/conversion system having an inlet for gaseous orgas-borne liquid fuel and oxidant (air). Other aspects are relevantinsofar as the energy release/conversion system has TVC features, aswill be addressed.

Among the varieties of energy release/conversion apparatus arecombustors and reaction chambers having a substantially steady-statevortex circulation in some aspect, situated at least in part off of adirect flow path from combustor inlet to exit. The term “trapped vortexcombustor” (TVC), used herein to denote one class of equipment to whichthe present disclosure is applicable, will be used as a most generalreference to energy release/conversion systems (both combustors and/orreformers) having such characteristics, and the internal portion of thecombustor containing the vortex will be referred to as the “trappedvortex reaction cavity”. A trapped vortex cavity may have contain onevortex flows, double vortex flows, or a plurality of vortex flows. Thetrapped vortex cavity may have continuously curved walls, or it may haverectilinear or other shaped walls, or be formed between bluff bodies, ora combination of walls and bluff bodies. A combustor or reaction chambermay also have a plurality of trapped vortex cavities. Examples of TVCembodiments in accordance with Roquemore, et al., Burrus, Haynes et al.and Steele et al. were previously discussed. The energyrelease/conversion system disclosed in Rakhmailov '854, though differentin material respects from those TVCs, should also considered a TVC forpurposes of the terminology employed in this disclosure.

Additional considerations can be relevant with regard to TVCs, dependingon reactor design. The vortex is often designed to help maintain flamestability of the combustor. Some designs further rely on using lowturbulence characteristics to help achieve uniform mixing, permittinglean mixtures to be burned at relatively low combustion temperatures,with consequent improvements in emissions. In practice, the operation ofan energy release/conversion system may be subject to fluctuations andperturbations, for example, as a result of discontinuities in fuel flowor compressor flow, small surface discontinuities, or statisticalanomalies resulting from small deviations from continuous and laminarflow as inevitably result when real fluids dynamically travel at highspeeds along real machine surfaces and chemically react with each otherin real time. Such fluctuations and discontinuities can lead to flameinstability from time to time. Thorough fuel and air mixing prior tocombustion (or reformation) can be used to improve the emissionsperformance and stability of such designs, as well as other designs inwhich turbulence may be permitted or promoted after introduction of thefuel-air mixture.

In TVC designs, premixers upstream of the main inlet have been employedto help mix fuel and air. However, experimentation has shown that thereis considerable room for improvement in the performance of these inletpremixer designs.

In certain TVC designs, fuel, air and/or premixed fuel and air have beeninjected directly into the combustion vortex to stabilize overallcombustor operation. For example, it has been the object of certain suchdesigns to use a vortex premixer, or discrete injection of fuel and/orair, to induce one or more additional vortices (where fewer or only onlyone vortex would otherwise exist), to create more mixing turbulence andincrease residence time of fluid in the TVC vortex cavity. However, suchdirect fuel or mixture injection, as previously practiced, wherein fuel,air and/or premixed fuel and air is injected disruptively to naturalvortex flow can lead to irregular mixing and hot spots and be suboptimalfor emissions control.

Thus, additional and different modes of premixing beyond those alreadypracticed in this field, could be advantageous, both for TVC (includinglow-turbulence TVC) and non-TVC designs. However, there are certainissues that must be overcome in order to successfully use a premixer.

An energy release/conversion system is typically fed combustion air in acompressed state, from a compressor. Premixed fuel and air under therelatively high pressures and temperatures that characterize compressoroutputs tend to be highly explosive. To avoid such an explosion,premixing in such an environment may be performed in a manner thatreduces the residence time of the premixed mixture prior to introductioninto the combustion chamber. This means that the premixing apparatusshould preferably be as close as practicable to the combustor inlet(short premixing length), while at the same time providing for adequateresidence time for the desired level of premixing (achievable in part byhaving small premixing scales). Accordingly, configurations forachieving fast and uniform premixing are preferred.

Furthermore, a premixer preferably should be compatible with theremainder of the system design. For example, in low-turbulence systems,the premixer should not rely on or introduce large-scale turbulenceinconsistent with the overall system design. In other TVC designs, thepremixer should introduce mixture in a manner consistent with thedesired cavity flow field.

With the foregoing principles and observations in hand, twocomplementary premixer designs have been developed. FIG. 1 is aperspective cross-sectional view from the inlet side to the exit side ofa combustor incorporating examples of these two different premixerdevices, 21 and 31, in accordance with certain embodiments of theinvention.

The combustor of FIG. 1, while axis-symmetric in internal configuration,is a “can” design (sometimes referred to as “can-annular”, but to bedistinguished from “full annular” designs), such that a plurality ofsuch “cans”, each as partially shown in FIG. 1, can be arranged in acircular configuration, with their outlet commonly directed around alarge turbine. Such arrangements are commonly used to power large-scalegas turbines for power generation applications, for example, where theshaft of the turbine drives a large electric generator. While FIGS. 1and 4 focus on a can design, those of skill in the art will recognizethat the principles of this design may be readily adapted to a “fullannular” combustor design, wherein a single, annular combustor isdesigned to mate with, for example, the annular fluid inlet of aturbine. Full annular designs may be used, for example, in commercialjet aircraft engines and auxiliary power units (APUs), in addition tolarge-scale power generation turbines. All of these configurations, andthe applications that they support, are within the scope of the presentinvention.

The can combustor in the embodiment shown in FIG. 1 has an inlet 6, anexit 5, and a recirculation zone 7, with a take-off point 14. In threedimensions, recirculation zone 7 forms a toroidal structure around theaxis of main flow from inlet 6 to exit 5, within which a vortex 22 willrecirculate during combustor operation, with a portion of the combustiongasses being taken off at point 14 and recirculated along and withincurved wall 23, around toroid 7 to rejoin inlet 6. Compressed combustionair is introduced at compressor inlet 10, upstream of inlet 6. Inletpremixer 21 is situated between compressor inlet 10 and combustor inlet6. In this embodiment, a generally conical bluff body 3 and 4 isprovided between premixer 21 and combustor inlet 6 to provide a nozzleto accelerate premixed mixture exiting premixer 21. (Note, however, thatin an annular design, the bluff body might be a “2D” element instead,extended annularly (as tapered ring) around the entire annular combustorinlet, instead of forming a cone-like structure as shown.)

In other particulars of this embodiment, bluff body 3, 4 terminatesagainst plate 11, which, in three dimensions, forms a circular wall inthe center of the inlet area 6; bluff body 3, 4 is supported by radialspokes 17, circumferentially arranged within inlet area 6; holes 16 areprovided in plate 11 for purposes of wall cooling (effusion, impingementeffusion, or the like); upstream the combustor inlet is pressure port12; and a mounting point 15 is provided for an igniter (not shown) inthe wall of the toroid. Further in this embodiment (i.e., a cancombustor), the combustor is housed within generally can-shapedenclosure 41.

In the illustrated embodiment, a second premixer 31 (vortex premixer) isalso provided. The second premixer injects into recirculation space 7,but differs in its approach from injection of premixed fuel and air aspracticed in the prior art. In prior art vortex premixers, premixed fueland air was introduced into the TVC cavity in a manner calculated eitherto create one or more additional and entirely new vortices, or tosubstantially disrupt the flow fields of a naturally existing vortex. Inthe illustrated embodiment, the injection of premixed fuel and air intothe vortex area is designed to enhance natural vortex flow in thecavity, for example, being introduced into the cavity along an exteriorwall, in a direction tangential to and codirectional with the naturalrecirculation flow in the cavity. Thus, it enhances the vortex flow,without disruption or significant added turbulence.

It should be understood that while FIG. 1 depicts both premixer 21 andpremixer 31 in place, either one of premixers 21 or 31 could be providedand the other eliminated, although as discussed later in this disclosurethere are operating modes in which it is advantageous to have bothpremixers.

In one embodiment, vortex premixer 31 comprises a number of fuelinjectors each arranged around the periphery of the outer wall of toroid7, directed tangentially relative to the direction of the combustorexit, near top dead center of toroid 7 and therefore approximatelytangential thereto, to inject premix mixture codirectionally with thenatural recirculation flow of fluid in toroid 7. Only one such injectoris shown in the sectional view of FIG. 1, but it should be understoodthat the injector structure is replicated by similar injectorsside-by-side around the periphery of toroid 7.

The two premixer systems will now be described in further detail.

The Inlet Premixer

FIGS. 2A, 2B and 3 show further detail on inlet premixer 21. In theembodiment reflected in premixer 21, fuel (which can be gaseous orliquid fuel) is introduced at fuel inlet 13 into central hub 1 andissues via the four radial spokes 19, etc. into the four concentricrings 2A, 2B, etc. mounted on the fuel spokes. Fuel is injected at anangle in the range from 0-90 degrees from the axial direction (eitherpositive or negative (i.e., either away from or toward the axis,depending on the specific ring pairing)), but in any case so that theabsolute value of the angle is in the range of from 0-90 degrees, andmore preferably 30-90 degrees from the axial direction) into the annularpassages formed between adjacent rings 2A, 2B, etc., into thehigh-velocity air from the compressor.

Concentric rings 2A, 2B, etc. and spoke 19 are aerodynamically shaped asshown in FIG. 2B. The number of rings should be in the range from 2 toabout 5, and should preferably provide less than 50% or more preferablyless than 40% flow blockage. Fuel orifices (holes) 201, 202, etc. inrings 2A, 2B, etc. are selected to provide adequate fuel penetrationinto the incoming air so as to provide the most thoroughly mixed(minimal unmixedness levels) mixture over the engine's operatingenvelope. Hence, they will be of different sizes/diameters, to targetvarious points on the load curve.

In an alternate embodiment, fuel can be fed to the spokes from theoutside diameter 212 of the ring structure, rather than the hub 211.

Preferably, holes 201 and 202 are directed to inject fuel in apredominantly radial direction, into the cross-flowing compressor air.In this embodiment, for ease of manufacture, the holes are directed atapproximately ±70 degrees relative to the axial direction. However, thisangle is not critical. In general, the range of angles can be from about±0 degrees to about ±90 degrees; is preferably between about ±60 andabout ±90 degrees; and more preferably about ±90 degrees.

The holes sizes are non-uniform, with different size holes provided forbetter performance in different power ranges, i.e., different fuel-airmomentum flux ratio levels. The various hole sizes can be determinedeither through cold flow mixing experiments, CFD or empiricalcorrelations, or any combination thereof. The total number of holes maybe greater than the corresponding number in a conventional premixer.

In one configuration, hole sizes are selected to provide a plurality ofholes in three different sizes, adapted to give the best mixtures inlow, middle and high power bands, respectively. For example, the smallholes may be sized for optimal low power operation (0-30% engine load);the medium holes for optimal medium power operation (0-70% engine load);and the large holes for optimal high power operation (70-100% engineload), and used in combination to provide the most uniform mixing overthe entire power range. Preferably, each hole is paired with a hole of adifferent size, on two adjacent rings, across the annular passagedefined by the two rings. Each of the aforementioned hole diameters(three different diameters in this example, though there could be agreater number) may be circumferentially distributed approximatelyevenly to assure the most uniform mixture within the cross-sectionalarea of the premixture inlet.

An exemplary hole layout is shown in FIG. 3. Angles χ1 and χ2 (not shownin the drawings) are radial angular hole offsets. The following tableshows, as one possible example, the hole layout for one quadrant, forthe eight rows of holes in the illustrated embodiment:

Row # Angle χ1 (First Hole) Angle χ2 (Spacing) # Holes 1 15 12 6 2 712.6 7 3 6 7.8 11 4 4.25 6.8 13 5 3.5 5.2 17 6 3.5 3.6 24 7 3.5 3.3 26 83 6.5 14

The combination of these features provides for reduced mixing length andscales, a wider envelope of mixedness uniformity over the engineoperating range, and is gas and liquid fuel compatible.

Prior art fuel-air mixers also include some hub-and-spoke designs aspreviously discussed. However, they do not inject fuel from concentricrings or provide a plurality of different injection hole sizes, and theydo not exhibit and/or achieve the same degree of mixing uniformity atany given engine load point. The prior art premixing strategy will notachieve the lower emissions performance of the present invention.

The inlet premixer described above achieves flat mixedness profiles andultra Low combustion exhaust levels for NOX, CO, UHC, etc. due to thehigher level of mixedness levels achieved by (a) nonuniform hole sizes,(b) a multitude of holes, and (c) smaller/shorter mixing scales. Thisprovides better mixing uniformity overall and over a greater engineoperating envelope. The approach described here can be tailored to anyengine fuel scheduling (i.e., how the fuel is metered over the engine'soperating envelope: fuel vs. engine load), and is gas or liquid fuelcompatible. It may be adapted to any combustor that uses liquid and/orgaseous fuel, for any application, including without limitation powergeneration, main propulsion jet engines for aviation (includingturbofans, turbojets, ramjets, etc.), and APUs.

Several months of combustion testing at several generic gas turbineengine operating load points has shown that this premixer, inconjunction with the other components described herein, can provideultra low emissions results (NOX, CO, UHC <3 ppm 15% O₂ simultaneously).

The Vortex Premixer

In one embodiment, as illustrated by vortex premixer 31 in FIGS. 1 and4, fuel and air are premixed prior to injection into the toroid/cavityflow. The injectors provide small mixing scales, a plurality ofinjection sites, and short required premixing length. Fuel and air arepremixed, or partially premixed prior to injection into the toroid. Thesecondary injection of the premix assists with general stability of thetoroid/cavity flow.

In the embodiment shown in FIG. 1, the natural flow in toroid 7, that isthe flow that would exist without premixture injection, for example as aresult of fluid flow through the main flow path of the combustor (frominlet 6 to exit 5), would be a single vortex 22 circulating from atakeoff point 14 proximate the exit, along the curvilinear wall of therecirculation space of toroid 7, and back to a point proximate the inlet6 of the combustor. In this embodiment, vortex cavity 7, thoughaccommodating a single vortex, is relatively large, lengthening theresidence time for combustion within the cavity. In a departure from theprior art, premixer 31 injects premixed fuel and air directly into therecirculation zone (trapped vortex cavity) 7 tangentially to wall 23,and thus aligned with the natural vortex flow 22 in the cavity, tointroduce the premixture with a minimum of disruption to the naturalflow pattern of vortex 22 and without substantial added turbulence.

As shown in FIG. 4, vortex premixer 31, in one embodiment, has aseparate fuel manifold 99 fed by tubular fuel inlet 39, in closeproximity to the air entry ports 8, which issues fuel into the incomingair flow through port 8 prior to injection within the cavity. In theembodiment shown, fuel manifold 99 encircles toroid 7, but there is onlya single fuel inlet 39 (although the inlet could be replicated in otherradial positions). Furthermore, in this embodiment, fuel manifold 99 isdivided by circular diffusion (DP) plate 38 having holes 42 etc. formetering fuel into submanifold 43, to reduce circumferentialnon-uniformities in fuel pressure in a single feed tube arrangement.Fuel flowing from submanifold 43 enters the incoming air flow (from 8),through fuel orifices (holes) 100, etc., mixes with the incoming air,and enters the toroid 7 through hole 24. The manner of premixing is notcritical but in the current embodiment will be a simple jet in crossflow. The application can be any fluid fuel type—liquid, gas, low BTU,Hydrogen enriched (“synthetic”) gas, etc.

As indicated by the plurality of holes 24 in FIG. 4, elements 8, 100 and24 are replicated circumferentially around the periphery of toroid 7,and fuel manifold 99 forms a cylindrical structure commonly feeding allthe premixer inlets. In this embodiment, recirculation vortex flow isfrom exit 5 to inlet 6, and incoming premixture is directed into toroid7 in a manner designed to result in smooth entry, approximatelytangential to and in about the same direction as, the recirculationvortex flow, with low turbulence. In one embodiment, there could beapproximately 100 or more such holes (24, etc.), evenly spaced aroundthe annular periphery of the outer wall of toroid 7, each having adiameter of a fraction of an inch, directed tangentially right beforethe point of extreme outer circumference of toroid 7. For example, inone embodiment, there could be approximately 100 or more injector portsarranged around the annular periphery of the outer wall of toroid 7,each having a diameter of about 0.1 inch to about 0.2 inch. In generalit is desirable to have a plurality of injector ports, for even andcontinuous premix introduction, and the greater the number of ports, thesmaller the port diameter (as scaled to the overall dimensions of thevortex container), with the higher numbers and smaller port sizes at anyparticular size scale being limited by fluid flow considerations asfriction increases due to small port diameters. In this manner, theincoming mixture enters smoothly, forming a boundary layer for vortex22.

The premix charge that is issued into the toroid through vortex premixer31 both strengthens/reinforces the toroidal flow and also provides ahigh combustion intensity pool of radicals to further stabilize the mainflow.

Premixer 31 can be located anywhere around the periphery of toroid 7,but, when recirculation is a single vortex moving in the direction fromexit to inlet, is preferably in the quadrant shown in FIG. 4, and morepreferably with the inlet close to point A in that Figure (i.e., closerto take off point 14) for a longer residence time.

The premixer-TVC combination shown in FIGS. 1 and 4 reflect how anembodiment of the present invention might be incorporated in a cancombustor. In large-scale power generation applications, a plurality ofsuch combustors may be deployed in a generally circular arrangement toexit into a gas turbine, in turn coupled to an electric generator.

The air entry ports 8 could also be adapted from the can configurationshown, to an annular configuration. The arrangement of ports in anannular configuration would be topologically similar to that shown inFIG. 4, dimensioned to fit around the periphery of an annular combustor.An annular combustor may also be attached to a gas turbine forlarge-scale power generation. In addition, an annular adaptation of theinvention can be applied, for example, to a main propulsion jet engineapplication for aviation (including turbofans, turbojets, ramjets,etc.), or to a smaller gas turbine/electric generator for use as an APU.Other applications include combustion turbines in an IntegratedGasification Combined Cycle (IGCC) plant, and in Combined Heat and Power(CHP) or cogeneration plants.

The vortex mixer described herein is not limited to TVC designs withcurvilinear walls as shown in FIGS. 1 and 4. For example, as shown inFIG. 5, a vortex premixer may be used with a TVC cavity havingrectangular walls 534 etc. TVC cavity 532 within combustor 501 as shownin FIG. 5 has a surrounding wall contour that is approximatelyrectangular on three sides. (In this embodiment, TVC cavity 531 isessentially a mirror image of TVC cavity 532 and set up similarly). TVCcavity 532 is shaped and dimensioned such that a single vortexapproximately indicated by arrow 533 will be established therein by mainflow 534 through combustor 501. Premixed fuel and air may be introducedin a manner that enhances the natural vortex flow in the TVC cavity, forexample at one or both of at one or both locations corresponding toarrows 510 and 509. In the case of arrow 510, the premixture injectionis approximately tangential to vortex 533 and enters substantially alongwall 534 adjacent to vortex 533. In the case of arrow 509, representingan additional premixer that may or may not be used in conjunction withthe one associated with arrow 510, injection is again approximatelytangential to vortex 533, and also substantially aligned with thedirection in which incoming flow 535 joins the flow of vortex 533.

A TVC design as shown in FIG. 5 may be used in a three-dimensionalrectangular structure (i.e., extending above and below the page of FIG.5), of which FIG. 5 would represent a cross-section. Alternately, theTVC design with such cavities may “wrap around” three-dimensionally inan annulus, to form an annular combustion chamber, with “upper” and“lower” toroidal vortex cavities having rectilinear walls, again with across-section corresponding to to the upper and lower cavity areas 531and 532 depicted in FIG. 5. The rectangular design could be used as acan combustor, and the annular design as an annular combustor, andrespectively deployed in applications similar to those described withrespect to the embodiment of FIGS. 1 and 4.

Fuel Staging

Although, as previously stated, premixers 21 and 31 can functionindependently, a combination of the two premixers, 21 and 31, describedhere, or a combination of inlet and direct-vortex injectors of othertypes, can be used in a fuel staging strategy in any TVC application.

Increasing the fuel intake through vortex premixer 31 relative to inletpremixer 21 can make operation more stable during engine turndown andpart-power application. Adding relatively equal amounts of air to both31 and 21 premixers permits superior turndown performance. This can beengine dependent, so these fractions can change. Typically, however, airthrough premixer 31 should generally be less than the air throughpremixer 21, or more narrowly, 20-40%.

More generally, fuel staging, as envisioned by one embodiment of thisinvention, entails injecting fuel at multiple and discrete locations inthe combustor to maintain flame temperatures in narrow bands, so thatNOX/CO/UHC emissions are below target rates. As the engine spools up,fuel is initially injected solely through holes 100 of vortex premixer31 until flame temperatures (φ) reach their NOX limit. See FIGS. 6A and6D. In a mid-power band (FIG. 6B), operation is switched so that fuelingis primarily through inlet premixer 21. In high power ranges (FIG. 6C),fuel is injected both through inlet premixer 21 and vortex premixer 31.Fueling discrete zones in this manner permits “walking’ up the powercurve while maintaining NOX and CO within prescribed limits, as requiredin view of flame temperature requirements. See FIG. 6E.

In addition, the combustor can be designed with a plurality of TVCcavities, each with separate premixer inlets. Injection into therespective TVC cavities can be varied relative to each other andrelative to inlet premixing, to provide finer control, and also toimprove temperature uniformity during turndown by allowing incrementalchanges in fueling through a sequence of cavities, rather than relyingon larger fueling changes in a single cavity.

With reference to the embodiment in FIGS. 1 and 4, preferably, vortexpremixer 31 is not completely turned off during operation. Rather, it ispreferable to phase the proportions of fueling from one premixer to thenext, while always keeping vortex premixer 31 at least minimally turnedon.

Other embodiments may involve a plurality of TVC cavities. For example,an embodiment as shown in FIG. 7A comprises two TVC cavities positionedaxially, upstream TVC cavity 702 and downstream TVC cavity 703, withcorresponding premixer inlets 742 and 743, as well as inlet premixer791.

A plurality of premixer-fed TVC cavities may include TVC cavities thatare radially positioned with respect to each other. One example of suchan arrangement is shown in FIG. 7B, showing radially arranged cavities712 and 713, and corresponding premixer inlets 752 and 753, as well asinlet premixer 792.

Each TVC cavity in multi-cavity arrangements, such as those illustratedby FIGS. 7A and 7B, may have its own vortex premixer inlet as shown (oralternatively, discrete fuel and/or air inlets), which can be fueledindependently of the other cavities and/or the main inlet. Sucharrangements provide a greater number of fuel-air zones and accordinglycan provide better turndown performance by spreading fueling changesacross a greater number of combustion areas.

In other embodiments, the TVC cavities may be situated solely in theinside of the combustor, as shown in FIGS. 7C and 7D. Such cavities may,for example, be curvilinear and axially staged, such as cavities 722 and723 in FIG. 7C, and corresponding premixer inlets 762 and 763 (as wellas inlet premixer 793), rectilinear and axially staged, such as cavities732 and 733 in FIG. 7D and corresponding premixer inlets 772 and 773 (aswell as inlet premixer 794), or other combinations of cavity shapes,locations and arrangement.

Preferably, in an axially arranged multi-cavity TVC embodiment, the mostupstream cavity (at a minimum), e.g., cavity 702 in FIG. 7A, would becontinuously fueled during operation, albeit at a variable level. In aradial configuration, preferably the largest cavity on a radius (at aminimum), e.g., cavity 712 in FIG. 7B, would be continuously fueledduring operation, again, at a variable level.

As stated above, the manner of operation described above is not limitedto the inlet and vortex premixers specifically depicted or described inthe present disclosure. While the principle of operation of “fuelstaging” as envisioned hereby is primarily described with particularreference to the premixer designs specifically taught herein, it shouldbe appreciated that any TVC that provides a combination of an inletpremixer and a direct vortex premixer can potentially benefit from thismethod.

The combination of an inlet premixer and one or more vortex premixers,which may be independently fueled, provides better optimization thanpreviously available for ultra low emissions performance. Each of thepremixers preferably provide for reduced mixing length and/or scales,result in a wider range of mixedness uniformity over the engineoperating envelope, and therefore superior emissions performance, aremulti fuel compatible (gas or liquid), can be tailored to any enginefuel scheduling and indeed are optimal for fuel staging applications.

For example, in an axial multi-cavity layout such as that shown in FIG.7A, one fuel staging strategy for turning up and turning down thecombustor might involve fueling through the first toroid premixer inlet742 from 0-33% power, backing off premixer 742 and fueling primarilythrough main inlet premixer 791 over the range of 33-66% power, and thenusing substantial flow though all three of premixers 791, 742 and 743for the range of 66-100% power. Similar as well as varied strategies maybe devised for any of the layouts depicted in FIGS. 1, 4, 6A-6C, 7A-7D,and other configurations. In some embodiments, the fuel controls for therespective premixers may be manually actuated; in other embodiments, thecontrols may be computerized, based on inputs such as power,temperature, NOX or CO concentration, time, etc., or may be computerizedwith manual overrides.

The fuel staging approach outlined above is not limited to use withpremixers having the structural characteristics discussed in connectionwith FIGS. 1 and 4. For example, Haynes et al. employs conventionalpremixers for both the main combustor inlet and the TVC cavity.Nevertheless, a similar technique to that described above could bepracticed with such a combustor to phase fuel and air flow from onepremixer to another, or to a combination of premixers, as best suitedoperation in various power bands.

Similarly, the utility of fuel staging as described above extends to alltypes of gas turbine applications requiring variable power output,including large frame power generation, aviation applications includingmain propulsion jet engines (including turbofans, turbojets, ramjets,etc.) and APUs, as well as to combustion turbines in an IntegratedGasification Combined Cycle (IGCC) plant, and in Combined Heat and Power(CHP) or cogeneration plants.

It is apparent, therefore, that the invention meets the objectives setforth above and provides a number of advantages in terms of ease of useand effectiveness, over the prior art. Although the present inventionhas been described in detail, it should be understood that variouschanges, substitutions, and alterations may be readily ascertainable bythose skilled in the art and may be made herein without departing fromthe spirit and scope of the present invention as defined by the claims.

I claim:
 1. A premixer in combination with a trapped vortex combustion(TVC) reactor, said premixer comprises an inlet premixer and a vortexpremixer; said vortex premixer is attached to said TVC reactor andcomprises a fuel inlet, an air inlet, a chamber wherein fuel and air aremixed, and an outlet for fuel-air mixture; said TVC reactor comprisingat least one trapped vortex cavity shaped and dimensioned such that themain flow of fluid through said reactor is sufficient to induce a vortexto circulate within said trapped vortex cavity irrespective of any flowof fuel-air mixture from said premixer; wherein said premixer ispositioned on said TVC reactor so that said outlet introduces saidfuel-air mixture directly into said trapped vortex cavity, and so thatsaid fuel-air mixture is introduced into said trapped vortex cavity atan angle tangential to the recirculation flow within the cavity suchthat said fuel-air mixture joins the flow of said vortex approximatelycodirectionally with said vortex flow; said inlet premixer comprising aplurality of concentric, coplanar, aerodynamically shaped ring elementspositioned upstream of said TVC reactor, axially aligned within a flowpath, each said ring element having an interior passage for fuel, eachsaid ring element further comprising a plurality of fuel injectionorifices whereby fuel flows from said interior passage into the inletfluid flow proximate to said ring, and wherein each pair of said ringelements defines an annular passage therebetween; and a conical bluffbody located directly downstream of said inlet premixer and upstream ofsaid TVC reactor configured to provide a nozzle and accelerate apremixed mixture exiting said inlet premixer.
 2. The premixer and TVCreactor of claim 1, wherein said trapped vortex cavity has a curved wallbetween an inlet and an outlet of said TVC reactor, wherein said vortexpremixer further comprises at least one injector tube orientedtangentially to said curved wall and extending from the outside said TVCreactor through said curved wall and into said trapped vortex cavity. 3.The premixer and TVC reactor of claim 2, wherein said TVC is a cancombustor, further comprising a plurality of said injector tubesarranged circumferentially around the radial periphery of said TVC. 4.The premixer and TVC reactor of claim 2, wherein said TVC is an annularcombustor, further comprising a plurality of said injector tubesarranged around the annular periphery of said TVC.
 5. The premixer andTVC reactor of claim 3 wherein the vortex flow in said TVC is from exitto inlet, and further wherein said circumferential arrangement ofinjector tubes are disposed proximate the midpoint of said combustor andextend toward said exit.
 6. The premixer and TVC reactor of claim 3,wherein said plurality of injector tubes is about 100 tubes, each ofsaid tubes having a diameter of approximately 0.1-0.2 inches.
 7. Thepremixer and TVC reactor of claim 1, wherein said trapped vortex cavityhas walls defining a rectilinear cross-section, wherein said vortexpremixer is further oriented to introduce said fuel-air mixture from atleast one inlet in said trapped vortex cavity, substantiallycodirectionally with said vortex.
 8. The premixer and TVC reactor ofclaim 7 wherein said TVC is an annular combustor, further comprising aplurality of said injector tubes arranged around the annular peripheryof said TVC.
 9. An assembly comprising a premixer and TVC reactor inaccordance with claim 1, further comprising a gas turbine, said assemblybeing adapted for electric power generation.
 10. An assembly comprisinga premixer and TVC reactor in accordance with claim 1, furthercomprising a gas turbine, said assembly being adapted for use as apropulsion jet engine for aviation.
 11. An assembly comprising apremixer and TVC reactor in accordance with claim 1, further comprisinga gas turbine, said assembly being adapted as an auxiliary electricpower generation unit.
 12. An assembly comprising a premixer and TVCreactor in accordance with claim 1, further comprising a gas turbine,said assembly being adapted as a combustion turbine for a combined heatand power plant.
 13. An assembly comprising a premixer and TVC reactorin accordance with claim 1, further comprising a gas turbine, saidassembly being adapted as a combustion turbine for an integratedgasification combined cycle plant.
 14. The premixer and TVC reactor ofclaim 1, wherein the vortex premixer outlet is disposed at thedownstream side of the TVC.
 15. The premixer and TVC reactor of claim 1,wherein fuel-air mixture is introduced through an opening extendingalong a curvilinear length of a trapped vortex cavity wall.